The invention relates to a method for determining the NOx concentration in a gas, in particular in the exhaust gas of an internal combustion engine. The concentration is determined with a measuring sensor having a solid electrolyte. The measuring sensor has a first measuring cell, into which the gas passes via a diffusion barrier, in which the oxygen concentration is measured via a first Nernst voltage between a first electrode and a reference electrode, exposed to the ambient air, and is controlled to a first oxygen concentration by means of a first oxygen-ion pumping current between the first electrode and an external electrode. The measured value of the first Nernst voltage is used for controlling the first oxygen-ion pumping current. The sensor further has a second measuring cell, which is connected to the first measuring cell via a diffusion barrier and in which the oxygen concentration is measured via a second Nernst voltage between a second electrode and the reference electrode and is controlled to a second oxygen concentration by means of a second oxygen-ion pumping current (Ip1) between the second electrode and the external electrode. The NOx concentration is measured at the same time by a measuring electrode in the second measuring cell.
For measuring the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to use a thick-film measuring sensor. Such a measuring sensor is described in the publication by N. Kato et al., xe2x80x9cPerformance of Thick Film NOx Sensor on Diesel and Gasoline Engines,xe2x80x9d Society of Automotive Engineers, publication 970858, 1997, or in N. Kato et al., xe2x80x9cThick Film NOx Sensor for the Measurement of Low Nox Concentrationxe2x80x9d, Society of Automotive Engineers, publication 98D170, 1998. That measuring sensor has two measuring cells and consists of a zirconium oxide that conducts oxygen ions. The system implements the following measuring concept: in a first measuring cell, which is fed the gas to be measured via a diffusion barrier, a first oxygen concentration is set by means of a first oxygen-ion pumping current, with no decomposition of NOx taking place. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is further reduced by means of a second oxygen-ion pumping current and NOx decomposes at a measuring electrode. The oxygen generated in this way is sensed as a measure of the NOx concentration. The entire measuring sensor is in this case brought to an elevated temperature, for example 430xc2x0 C., by means of an electric heater. The measuring error of the measuring sensor described in the publication corresponds to an NOx concentration of 22 ppm.
The measuring error is of a systematic nature for the MO following reasons: for setting the first oxygen concentration in the first measuring cell, the oxygen-ion pumping current Ip0 flows between a first electrode and an external electrode through the thick-film material, which in this case is a solid electrolyte of ZrO2. Between the solid electrolyte ZrO2 and the first electrode there is a transitional resistance R, through which the oxygen-ion pumping current Ip0 flows. The resultant voltage drop is measured at the same time as the determination of a first Nernst voltage V0 in the first measuring cell, which is used for controlling the first oxygen-ion pumping current Ip0. The measured voltage V0 thus comprises the actual Nernst voltage VN0 and the described additive voltage drop across the transitional resistance R0. This makes it more difficult for controlling to the first oxygen concentration to be carried out in the first measuring cell and leads to a systematic error, since too low a first oxygen concentration may under certain circumstances cause NOx that was actually intended to be decomposed and measured only in the second measuring cell to be decomposed already at the first electrode. If the first oxygen concentration is too high, too much oxygen diffuses into the second measuring cell, in which the residual oxygen content becomes too high as a result. This likewise leads to a falsification of the NOx measurement.
In the prior art, this problem has been solved by using the fact that, with an ideally set first oxygen concentration at the first measuring cell, the setpoint value for the second oxygen-ion pumping current Ip1, with which the second oxygen content in the second measuring cell is set, is known. This setpoint value for the second oxygen-ion pumping current Ip1 can then be used for correcting the measured value V0 of the first Nernst voltage to the extent that the second oxygen-ion pumping current Ip1 achieves this setpoint value. The control required for this cannot, however, be designed to be very fast, for reasons of stability, so that when there are rapid changes in NOx concentration in the gas to be measured compensation is not completely possible. Then the first oxygen concentration in the first measuring cell is not controlled to the desired value, which, as described, leads to measuring errors in the sensing of the NOx concentration.
It has become known from U.S. Pat. No. 6,036,842 (see Japanese application JP 170160/96 and European published application EP 0 816 836 A2) to incorporate a voltage derived from the first pumping current into the control of the first pumping current for correcting a transitional resistance.
The object of the invention is to provide a method for determining a NOx concentration in a gas which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which allows a more exact sensing and measurement of the NOx concentration in a gas using the above-described measuring sensor.
With the above and other objects in view there is provided, in accordance with the invention, a method of determining a NOx concentration in a gas, in particular in an exhaust gas of an internal combustion engine. The method comprises:
providing a measuring sensor with a solid electrolyte, a first measuring cell with a first electrode, a second measuring cell with a second electrode, a diffusion barrier separating the first measuring cell from the second measuring cell, and a reference electrode exposed to ambient air;
allowing a gas to pass via a diffusion barrier into the first measuring cell, measuring an oxygen concentration in the first measuring cell via a first Nernst voltage between the first electrode and the reference electrode, and regulating the oxygen concentration to a first oxygen concentration with a first oxygen-ion pumping current between the first electrode and an external electrode, and thereby using a measured value of the first Nernst voltage for controlling the first oxygen-ion pumping current;
measuring an oxygen concentration in the second measuring cell via a second Nernst voltage between the second electrode and the reference electrode, and regulating the oxygen concentration in the second measuring cell to a second oxygen concentration with a second oxygen-ion pumping current between the second electrode and the external electrode;
also measuring the NOx concentration with a measuring electrode in the second measuring cell; and
correcting an error occurring in a setting of the first oxygen-ion pumping current by correcting the measured value of the first Nernst voltage with a product of a correction value and the first oxygen-ion pumping current, wherein the correction value approximates a transitional resistance between the first electrode and the solid electrolyte being supplied at least partially by a controller, wherein the controller uses as a reference variable a prescribed value (Ip1set) of the second oxygen-ion pumping current and as a controlled variable a current value of the second oxygen-ion pumping current.
In other words, the measured value V0 of the first Nernst voltage is corrected by a controller, which uses as the reference variable a setpoint value Ip1set of the second oxygen-ion pumping current and as the controlled variable the current value of the second oxygen-ion pumping current Ip1 and, as a result, approximates the transitional resistance R0 at least partially by a correction value R0xe2x80x2. Since the transitional resistance R0 is subjected to only slowly changing influencesxe2x80x94to be mentioned here in particular are the temperature of the measuring sensor, the temperature of the gas to be measured or the mass flow of the gas to be measuredxe2x80x94this controller can be designed in such a way that it operates only very slowly, and as a result is very exact. The voltage drop, falsely measured at the same time as the determination of the first Nernst voltage UNO, across the transitional resistance R0 between the solid electrolyte and the first electrode can then be corrected by the product of the correction value R0xe2x80x2 and the current Ip0, since the first oxygen-ion pumping current Ip0 is known.
The advantage of this procedure is that an exact correction of the measured value V0 for the first Nernst voltage is now possible even when there are rapid dynamic changes in the gas to be measured, since there is a single correction value R0xe2x80x2 of the resistance R0 for the various operating states. After all, the correction value R0xe2x80x2 for the resistance is used only to compensate for a slow change. Consequently, the accuracy of measuring the NOx concentration is increased.
In accordance with an added feature of the invention, an approximation of the transitional resistance by the correction value is partly supplied with the controller and partly taken from a characteristic map that depends on at least one of the following variables: a temperature of the measuring sensor, a temperature of the gas to be measured, and/or a mass flow of the gas to be measured.
In accordance with an additional feature of the invention, the transitional resistance is shifted by a correction value composed of a constant value and a value supplied by the controller.
In accordance with a concomitant feature of the invention, the constant value is assumed only if the temperature of the second measuring cell and the temperature of the first measuring cell are substantially equal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining NOx concentration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.